1. Field of the Invention
This invention relates to gas turbine engine turbine dimensional restoration repair using air plasma thermal metallic spray processes and, more particularly, to such turbine rotor repairs having localized compressive residual stresses imparted by laser shock peening to the substrate.
2. Description of Related Art
A gas turbine engine includes a fan section, compressor section, a combustion section and a turbine section. Disposed within the turbine section are alternating annular stages of circumferentially disposed moving blades and stationary vanes. The rows or stages of vanes and blades are concentrically located about a center-line axis of the gas turbine engine. The blades are mounted on a disk which rotates about its central axis. Hot combustion gases exit the combustor and pass through the turbine section, whereupon the blades which are mounted on the disks rotatably drive a shaft, thus, providing shaft work for driving the fan and compressor sections and auxiliary systems. Higher gas temperatures allow more work to be extracted from the gases in the turbine section, thus, increasing the overall efficiency of the gas turbine engine.
The assembly of rotating components of a gas turbine engines, including aircraft gas turbine engines, is generally referred to as the rotor and operates at high rotational speeds that subject the rotor's components to very high tensile stress fields, particularly, at radially outer edges of the rotors. Furthermore, portions of the rotor that operate in the turbine are subject to high temperature operating conditions. Cobalt and nickel-base superalloy materials have been developed to provide mechanical strength at high temperatures so that the operating temperature capability of the turbine section is increased over the operating temperatures of prior designs. Rotor components are constructed to be highly resistant to deformation at high temperature are usually produced by various types of forging process. The material and processes shapes are very costly because of the nature of the alloys and the complexity of the shapes of the components. The cost of such forgings are high and, thus, it is highly desirable to be able to repair the forging and components should any of the components be incorrectly machined or otherwise damaged during handling or operation. In spite of the development of casting operations and procedures which require close control and supervision components are still damaged during production and during operation, particularly, due to wear. Damages to forgings during production include nicks and dings due to handling and mismachining. These damages require repairs to avoid substantial amounts of waste and component replacement.
Gas turbine engine hot section components are particularly subject to damage or degradation in operation. This may be in the form of dimensional losses due to erosion, oxidation or corrosion, or distortion. When an engine is overhauled, these components are either replaced by new parts, which is very expensive, or repaired, which is clearly more desirable if a cost savings may be achieved. Several methods have been devised in which a metal is deposited by spraying onto a substrate to form an article of more complex shape, but the mechanical properties of such deposited layers of material have been in general much poorer than those of the corresponding wrought material. Similar methods are used to restore damaged or worn metal articles but, in this case, there is an additional problem in that the air plasma thermal metallic spray process causes low cycle fatigue life loss in the substrate below the deposited layer. This degrades the effectiveness of the repair.
To this end, a number of repair methods have been developed utilizing air plasma thermal metal spraying, brazing and welding for dimensional restoration. The air plasma thermal metal spraying process most frequently used involves a plasma heat source and powder consumables. However, such air plasma thermal metal spraying processes of the past have many limitations. Though such air plasma thermal metal spraying is most suitable for providing relatively thin coatings, problems and difficulties occur when it is used to repair relatively large geometric dimensions. Air plasma thermal metal spraying produces overlays that are mechanically bonded to substrates, do not have the structural integrity of the original forged component, and potentially degrade the fatigue life of the substrate. Thus, even though satisfactory for many repairs, it is highly desirable to develop better repairs that do not degrade the low cycle fatigue properties of the parent metal substrate and also may have higher mechanical bond strength to the substrate.
One repair method, disclosed in U.S. Pat. No. 4,878,953, entitled "Method of Refurbishing Cast Gas Turbine Engine Components and Refurbished Component", provides a source of background information related to methods for refurbishing cast gas turbine engine components and, particularly, for components made with nickel-base, cobalt-base and iron-base superalloys for use in the hot sections of gas turbine engines and, more particularly, for components exposed to high temperature operating conditions. U.S. Pat. No. 4,224,356, entitled "Deposition of Metals on a Base" discloses prior art methods for improving mechanical properties of a deposited metal by subjecting it to peening after deposition by bombarding the metal surface with relatively hard shot, thereby, effecting a cold working of the metal in the surface region thereof. Among the disadvantages noted by the inventor are internal compressive stresses, which are generated by the cold working action of the subsequent peening and, therefore, the patent suggests simultaneously spray depositing metal onto a substrate, whilst at the same time bombarding the deposit with hard rounded particles, so that the deposit is hot plastically deformed as it is being built up in order to provide "greatly enhanced physical and mechanical properties". This method involves complex and expensive equipment and processes.
The present invention is directed at all of the concerns and disadvantages noted above for using air plasma metal spray or deposition on a substrate of a metallic component in order to build up the component or to provide dimensional restoration. The present invention provides a dimensional restoration method and article, and gas turbine engine rotor component in particular, having a substrate with regions of deep compressive residual stresses imparted by laser shock peening over an area of the substrate upon which at least one layer of metal is deposited by air plasma or other metal spraying method.
The region of deep compressive residual stresses imparted by laser shock peening of the present invention is not to be confused with a surface layer zone of a work piece that contains locally bounded compressive residual stresses that are induced by a hardening operation using a laser beam to locally heat and, thereby, harden the work piece such as that which is disclosed in U.S. Pat. No. 5,235,838, entitled "Method and Apparatus for Truing or Straightening Out of True Work Pieces". The present invention uses multiple radiation pulses from high power pulsed lasers to produce shock waves on the surface of a work piece similar to methods disclosed in U.S. Pat. No. 3,850,698, entitled "Altering Material Properties"; U.S. Pat. No. 4,401,477, entitled "Laser Shock Processing"; and U.S. Pat. No. 5,131,957, entitled "Material Properties". Laser peening, as understood in the art and as used herein, means utilizing a laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface. Laser peening has been utilized to create a compressively stressed protection layer at the outer surface of a workpiece which is known to considerably increase the resistance of the workpiece to fatigue failure as disclosed in U.S. Pat. No. 4,937,421, entitled "Laser Peening System and Method". However, the prior art does not disclose a metallic layer sprayed onto a laser shock peened surface area of a metallic substrate having a region with deep compressive residual stresses imparted by laser shock peening extending into the substrate from the laser shock peened surface of the type claimed by the present patent nor the methods of how to produce them. It is to this end that the present invention is directed.